In holographic information storage, an entire page of information is stored at once as an optical interference pattern within a, photosensitive optical material. This is done by intersecting two coherent laser beams within the storage material. The first, called the data beam or signal beam, contains the information to be stored; the second, called the reference beam, is designed to be simple to reproduce—for example, a simple collimated beam with a planar wavefront.
The resulting optical interference pattern causes chemical and/or physical changes in the photosensitive medium: a replica of the interference pattern is stored as a change in the absorption, refractive index, or thickness of the photosensitive medium. When later illuminated with one of the waves used to create it, the interference pattern causes some of the incident light to be refracted such that the second wave is recreated. Thus, illuminating the interference pattern with the reference wave reconstructs the data beam, and vice versa.